The present invention relates generally to extrusion blow molding, and more particularly to a technique for extrusion blow molding of microcellular polymeric material.
Polymeric extrusion blow molding is a known process in which a molten polymeric material is extruded from an extruder die as a parison (an essentially cylindrical polymeric sleeve). The parison is placed in a mold and, typically while still warm enough to be soft and moldable, is subjected to significant gas pressure internal of the cylinder and expanded against the mold. Many common articles such as beverage bottles, motor oil bottles, pharmaceutical packaging, cosmetic packaging, and the like are manufactured using this technique.
In many cases, a parison is extruded so as to have differing thickness along its length. Thicker portions may correspond to locations where the article needs to be reinforced to a relatively greater extent, or to provide for expansion in some regions to a greater extent than in other regions (in the blow-molding formation of, for example, a plastic detergent bottle), while maintaining an essentially constant thickness in the molded article.
Foamed polymeric materials are well known, and can be produced by introducing a physical blowing agent into a molten polymeric stream, mixing the blowing agent with the polymer, and extruding the mixture into the atmosphere while shaping the mixture. Exposure to atmospheric conditions causes the blowing agent to gasify, thereby forming cells in the polymer. Alternatively, a chemical blowing agent can be added and caused to react in the molten polymeric stream, resulting in the generation of gas that forms cells in the polymer. In both cases, nucleating agents are normally used to control cell size and uniformity.
U.S. Pat. No. 4,444,702 (Thomas, et al.) describes a system for producing tubular extruded parisons of thermoplastic material, the wall thickness of the extruded parison being varied during extrusion.
U.S. Pat. No. 3,939,236 (Hahn) describes a technique involving extruding a cellular polymeric tubular parison, then blow molding the parison.
U.S. Pat. No. 3,225,127 (Scott) describes a process involving extruding molten plastic containing a foaming agent through an annular orifice to form a foamed parison, then placing the parison in a blow mold cavity and expanding the parison within the mold.
U.S. Pat. No. 4,874,649 (Daubenbxc3xcchel, et al.) states that major difficulties exist in extrusion blow molding of foam articles in which a preform that has already been foamed is expanded. Daubenbxc3xcchel, et al. state that foamed material of a preform that is still in a thermoplastic condition has regions that exhibit different strength and expandability values over the length and periphery of the preform, with the result that weak points are formed under the effect of internal pressure within the preform, and that in many circumstances these weak points cause the wall of the preform or the molded article produced therefrom to tear open, giving rise to wastage. Daubenbxc3xcchel, et al. purportedly solve this problem by co-extruding a multi-layer thermoplastic preform in which at least one layer is non-foamable. Using a non-foamable layer purportedly allows the preform to be expanded, after the material has been foamed, without giving rise to the danger of forming weak points or holes through the wall of the article. When the layer of non-foamable material is arranged on the outside of the article, an article is produced having a smooth exterior surface. Daubenbxc3xcchel, et al. also describe blow-molding expansion of the preforms at a pressure on the order of 1 bar, or less than around 0.5 bar, which they characterize as markedly lower than in the case of conventional extrusion blowing process, to avoid bubbles or pores in the foamed material from being compressed.
While processes for the extrusion blow molding of foamed polymeric material are known, a need exists for simplified processes for production of extruded blow-molded products having good physical qualities. It is an object of the invention, therefore, to provide extrusion blow-molded foam articles of good physical properties, and techniques for producing these articles. It is another object to provide relatively thin-walled extruded, blow-molded foam articles and techniques for producing these articles that involve controlling foam uniformity and density.
The present invention provides a series of articles, systems, devices, and methods associated with foam, blow-molded articles.
In one aspect, the invention provides an article. In one embodiment, an article is provided comprising a blow-molded, foam, microcellular, polymeric article.
In another embodiment, the invention provides an extruded, microcellular parison suitable for blow-molding.
In another aspect, the invention provides systems. One system includes extrusion apparatus having an extruder with an inlet designed to receive a precursor of polymeric microcellular material, constructed and arranged to provide a single-phase, non-nucleated solution of polymeric material and a blowing agent. A blow-molding forming die is fluidly connected to the extruder and has an outlet designed to release a parison of microcellular material. The apparatus includes an enclosed passageway connecting the extruder inlet to a blow molding forming die outlet. The passageway includes a nucleating pathway having length and cross-sectional dimensions selected to create, in a single-phase, non-nucleated solution of blowing agent and fluid polymeric material, a pressure drop at a rate sufficient to cause microcellular nucleation. A blow mold also is included, and is positionable to receive a parison of microcellular material from the die outlet.
In another embodiment, a system is provided that includes an extruder constructed and arranged to provide a polymeric foam precursor material, and an accumulator associated with the extruder. The accumulator is able to receive polymeric foam precursor material from the extruder and to accumulate a charge of polymeric foam precursor material. Blow molding apparatus also is provided in this system, and is positionable to receive a product of the accumulator, via a forming die. The blow molding apparatus is constructed and arranged to blow mold the material to form a blow-molded foam polymeric article.
In another embodiment a system that includes a combination of some aspects described above as provided. The system includes an extruder having an inlet to receive a precursor of polymeric microcellular material that is constructed and arranged to provide a single-phase non-nucleated solution of polymeric material and a blowing agent. An accumulator is provided and is positionable to receive polymeric foam precursor material from the extruder and to accumulate a charge of the polymeric foam precursor material. A blow-molding forming die is fluidly connected to the accumulator and has an outlet designed to release a parison of microcellular material. A blow mold is positionable to receive a parison of microcellular material from the die outlet and is constructed and arranged to form a blow-molded, foam, microcellular, polymeric article. The apparatus includes an enclosed passageway connecting the extruder inlet with the die outlet, the passageway including a nucleating pathway defined above.
In another aspect, the invention provides a forming die device. The die includes an inlet at an upstream end constructed and arranged to receive a single-phase, homogeneous solution of polymeric material and a blowing agent that is a gas under ambient conditions, and an outlet at a downstream end thereof, defining a die gap, for releasing foamed polymeric material. A fluid pathway connects the inlet with the outlet and includes a nucleating pathway. The die is constructed and arranged to vary the width of the die gap during extrusion while maintaining a constant nucleating pathway gap.
In another aspect, the invention provides a series of methods. In one embodiment, a method is provided that involves extruding polymeric foam extrudate from an extruder die while varying the thickness of the extrudate.
In another embodiment, a method is provided that includes providing an extrudate polymeric microcellular foam parison and subjecting the parison to blow molding conditions.
In another embodiment, a method is provided that involves extruding a polymeric foam extrudate from an extruder die in a machine direction while varying the temperature of the extrudate exiting the die. An extrudate thereby is formed having a first portion and a second portion spaced from the first portion in the machine direction, the first portion and second portion differing in material density by a factor of at least 1.1.
In another embodiment, a method is provided that involves subjecting a foam polymeric parison to relatively severe blow-molding conditions while maintaining relatively constant density in the parison. A parison can be subjected to blow-molding conditions of at least about 15 psi thereby expanding at least a portion of the parison at least about 50% in circumference. This takes place while the density of the parison remains relatively constant, in particular the density is increased by no more than about 20%.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.